Olefin polymerization using Ziegler-Natta type catalysts has been widely utilized in industry. These catalysts provide polyolefins in high yield possessing the desired physical characteristics of these polymers. However, the use of Ziegler-Natta catalysts do suffer some serious limitations. Thus, new and improved catalysts are continually being sought and developed. An important class of catalysts where improvement is sought is for the polymerization of the very commercially important alphaolefin, propylene.
Commonly in the polymerization of propylene a catalyst containing a magnesium halide support is utilized. However, when propylene is polymerized from such a supported catalyst, the polypropylene molding machine processing this polymer is subjected to extensive corrosion, or other damage, directly caused by the presence of residual halide in the polypropylene product. More importantly, the polypropylene molded article processed in this manner is characterized by aesthetic flaws.
Another serious drawback of art recognized catalysts, in the polymerization of propylene polymers, involves the required use of internal electron donors to insure that the propylene polymer product is highly isotactic. However, unless the amount and type of electron donor compound is carefully selected, not only is the stereoregularity of the resultant polymer deficient but poor catalytic activity often results. This detrimental effect will still occur even if the catalyst is formed with the roper electron donor compound added in the wrong sequence.
The utilization of electron donor compounds often creates additional problems involving the evolution of offensive odors in the final polymeric product. This offensive odor will also be present even if the ideal electron donor compound, in the correct concentration, added at the proper time in the catalyst formation process, is utilized. Thus, polymers formed from these catalysts which include an electron donor compound must oftentimes be deashed or deodorized in order to insure that the final product does not give off any offensive odors.
These difficulties discussed herein has lead workers skilled in the art to produce and develop new catalysts which are aimed to overcome these difficulties.
Drying of catalyst components, especially catalytic supports, is common in the industry. For the supports alone, calcining which implies heating under a nitrogen atmosphere is typically employed to control the presence of adsorbed moisture with these hygroscopic materials, at least in part to avoid hydrolytic reactions with other catalyst reactants. Moisture present in the support is widely recognized as one of several substances which may act as catalyst poisons. For these very reasons, however, such drying is carried out under stringent conditions to essential dryness. See e.g. U.S. Pat. No. 4,593,079.
Short et al. U.S. Pat. No. 4,565,795 discloses an olefin polymerization catalyst which is prepared by the reaction of a chemically treated silica support with a dihydrocarbyl magnesium compound and a halogenated tetravalent titanium compound The solvent for the dihydrocarbyl magnesium compound was removed by evaporation.
U.S. Pat. No. 4,530,913 of Pullukat et al. discloses a method of preparing a solid polymerization catalyst component by reacting a modified support with an organomagnesium compound and a tetravalent titanLum compound with no intermediate drying step. The solvent was subsequently evaporated.
Tachikawa et al. U.S. Pat. No. 4,686,199 describes a catalyst component formed from the reaction of a metal oxide with one or more non-halogenated magnesium alkoxide, a halogen-containing compound, and a titanium compound. Excess inert solvent associated with the magnesium compound is removed in Example 1 under nitrogen by vaporization, and after contact with another magnesium compound the solid was dried in vacuo at 90.degree. C. for 2 hours.
McDaniel et al. U.S. Pat. No. 4,855,271 describes a catalyst component prepared by contacting a support comprising highly porous alumina containing a magnesium compound with an alcohol and subsequently with a halide, alkoxide, or haloalkoxide of titanium and/or vanadium. Once the magnesium compound is impregnated onto the alumina support under the conditions of incipient wetness, drying conditions are selected so as to minimize reduction in the porosity of the support. Typically, this involves heating at a temperature of about 30.degree. C. to about 800.degree. C. More preferably, the drying temperature is from about 100.degree. C. to about 400.degree. C.
By way of background to the invention, U.S. Pat. No. 4,950,631 including inventor Buehler discloses a process for propylene polymerization using a modified silica based catalyst. The catalyst is prepared by first pretreating the silica support either by calcination or chemical treatment with a disilazane compound followed by modifying the silica by reacting it with a hydrocarbon soluble magnesium compound. In this case, the solid product was allowed to settle and the supernatant siphoned off. This modified support is then made chemically active by reacting with a titanium compound having the formula, Ti(OR).sub.n X.sub.m, wherein R is an aryl, alkyl or any mixtures thereof containing C.sub.1 -C.sub.28 atoms, X is a halogen, n=1 to 4 and m=0 or 1 to 3 such that n+m=4. This catalyst precursor is then reacted with another titanium compound having the formula TiX.sub.p '(OR').sub.q where X' is a halogen, R' is an aryl or alkyl group, p=1 to 4; and q=0 or 1 to 3 such that the sum of p and q equals 4. The final catalyst was dried by evaporation to a free flowing solid. Additionally, cocatalyst containing an aluminum compound and/or a silane compound can be used.
The significance of the drying and washing steps in relation to solvents in these catalysts became increasingly apparent to the present inventor(s), such that in copending and commonly assigned U.S. Ser. No. 590,992 of Buehler, Fries and Pullukat filed Oct. 10, 1990 incorporated herein by reference, a "minimum level" of solvent for the magnesium compound is mentioned This application also observes at page 23, lines 28-33 that underdrying or overdrying may be deleterious and expresses preference for drying after reaction with the magnesium compound at 70-80.degree. C. for 11/2 to 21/2 hrs. Then, in copending and commonly assigned U.S. Ser. No. 717,956 of Buehler and Masino filed Jun. 20, 1991 incorporated herein by reference, a drying step is disclosed in Example 19 to remove 67% by weight of the solvent from the magnesium compound.
It has been surprisingly discovered upon further consideration and elucidation of the parametric preparative conditions that drying to a selected solvent level, as it controls the nature of micro-deposition in and upon the surface of relatively high surface area, high pore volume inorganic supports, with applied hydrocarbon soluble magnesium compounds is a key variable in providing desired catalyst characteristics and in turn improved polymer properties in preparation and use. In accordance with the invention, simplified preparation of highly active propylene polymerization catalysts is afforded without internal electron donors, magnesium halide supports, or multiple reactants.